Transcription of Programming Logic controllers
1 Programming Logic controllersProgrammable Logic Controller (PLC) is a microprocessor based system that uses programmable memory to store instructions and implement functions such as Logic , sequencing, timing, counting and arithmetic in order to control machines and Features PLC features can be concluded in the following: They are rugged, withstand industrial environment, such as heat, humidity, mechanical shocks and vibrations The interfacing for inputs and outputs is inside the controller They are easily programmed PLC is capable of both Logic and PID control.
2 Typical PLCF orms of PLCs First developed in 1968, they are now widely used: Two types: 1- Single box type:for small programmable controllers , is supplied as an integral compact package, complete with power supply, processor, memory and input/output units. Typically they may have 6,8, 12, or 24 inputs and 4, 8, or 16, outputs and a memory store some 300-1000 instructions. Eg. MELSEC FX3U 2- Rack mounted type: for all sizing of Programming controllers . It consists of separate modules for power supply, processor, input/output, etc. They are mounted on rails with a metal cabinet.
3 SIMATIC S7-300/400 Input/output processing PLC is continuously running through its program and updating it as a result of the input signals, each such loop is called a cycle. Two methods of processing: 1- Continuous updating 2- Mass updating continuous updating: The cpu scanning the input channels as they occur in the program instructions. Each input is examined individually (delay time 3 ms). The output is latched so that they retain their status until the next updateMass input/ output copying: it works in the following process:1. Scan all the inputs and copy into RAM 2.
4 Fetch and decode and execute all program instruction in sequence, copying output instruction to RAM3- Once the program is executed, the CPU performs diagnostics and communication tasks4- update all outputsRepeat the processingInput/Output address The inputs and outputs are identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC. Its just a number preceded by a letter to indicate whether it is an input or an output With large PLCs having several racksof input and output and a number of modules in each rack, the rack and modules are numbered and so an input or output is identified by its rack number followed by the number of the module in that rack and the number to show its terminal in the module.
5 :The Allen-Bradley PLC-5 has I: 012/03to indicate an input in rack 01at module 2and terminal 03 Communication ModuleMicroprocessorInput ModuleExternal ComputerPLC In Control Circuits ArchitecturePLCO utput ModuleActuatorProcessSensorArchitecture of a PLCIt consist essentially of:Central processing unit: control all processes with frequency 1-8 M HzMemory: Buffers as temporary storage, ROM for system data & RAM for user programInput/output interface:System busesInput/output unit The input/output unitprovides the interface between the system and the outside world.
6 The input/ output interfaceprovides isolation and signal conditioning functionsso that sensors and actuators can often be directly connected to them without the need for other circuitry. Out devices: motors, starting coils, solenoid valve, etc Input devices: temperature sensors, flow sensors, encoders, channelInput/output unitElectrical isolationfrom the external world is usually provided by means of optisolators. Figure shows the basic form of input channel The digital signal that is compatible with the microprocessor of the PLC is 5 volt dchowever, signal conditioning in the input channel with isolation enables a wide range of input signals to be supplied.
7 Possible input voltages 5V, 24V, 110V, and 240V. The output to the output unit is digital with a level of 5 V, Three types of outputs are available: Relay type, Transistor type & triac typeInput/output unit Three types of outputs are available: Relay type: The signal from the PLC is used to operate a relay and so able to switch currents of a few amperes in an external circuit. The relay isolate the PLC from external world, can be used for AC and DC but they are slow Transistor type: It is used to switch current through external circuit, fast opto-isolators are used to provide isolation, used only for DC switching Triac type: used for both AC and DC Possible output from output channels: 24V,100mA; 100V dc,1 A; 240V,1A ac.
8 Or 240V, 2A acInput/output unitInputting ProgramsAre entered into the input/output from:Small hand- held Programming devices, desktop consoles with a visual displayor key board and screenInputting ProgramsOnly when the program has been designed and checked on the Programming device is transferred to the memory of the PLCT ypical Program memory size is (specified in term of steps) 300 to 1000 step is an instruction for some event to occur, ex: check status of switch AAfter developing the program in RAM it may be transferred permanently to the EPROM chip Figure (a), (b) Sourcing, (c), (d) sinkingFigure (a), (b) Alternative ways of drawing an electric circuit,(c) comparable rung in a ladder programLadder programmingThe form of Programming commonly use with PLC is ladder Programming .
9 Each program task is specified as though a rung of a a rung could specify that the state of switches A and B be examined and if both A and B are closed then a solenoid, the output is sequence followed by a PLC when carrying out a program:1- Scan the inputs associated with one rung of the ladder program2- solve the Logic operation involving those inputs Set/ reset the outputs for that rung3- move on to the next rung and repeat operations 1, 2, so on until the end of program with each rung of the ladder scanned in turn. The PLC then goes back to the begining of the program and startsagain The ladder diagram consists of two vertical lines representing the power (rung)
10 Are connectedas horizontal lines,Ladder programmingLadder shows a basic standared symbols that are used and rung,Inputs must always preceede outputs and there must be at least one output on each lineEach rung must start with an inputor series of inputs and end with an outputFigure controlling a solenoidExample of a ladder diagramLadder programmingThe output from the PLC is to energise a solenoid when a normally open start switch connnected to the input is activated by being closedThis might be a solenoid valve which opens to allow water to enter a control systemThe input goes from low to high when the temparature sensor reaches the set temperature.
